Review and Preview of Alkenes and Alkynes

3. Review and Preview • Alkenes and alkynes are hydrocarbons • Newman projections illustrate conformations • Cycloalkanes use the cis/trans nomenclature. • Physical properties of alkanes and cycloalkanes are similar to each other. • Alkene and alkyne stability relate to their structure and degree of substitution. • Pi bonds in alkenes and alkynes gives them types of reactivity that alkanes lack

4. As You Study… As you study, here are some concepts and skills of particular importance: • How do alkanes, alkenes, and alkynes differ from one another? • What orbitals are involved in forming bonds in each class of compound? • What configurational isomers are possible for alkenes and how do we provide names that distinguish these isomers from one another? • What aspects must be considered when thinking about ring compounds that incorporate double or triple bonds? • What effect do substituents have on the relative stabilities of alkenes and alkynes, and how are relative stabilities measured?

5. Alkanes and Alkenes • Alkanes have the formula CnH2n+2. • Cycloalkanes with one ring have a different formula: CnH2n. • A second family of hydrocarbons also has the formula CnH2n. These are known as alkenes. • Alkynes are a third family of hydrocarbons having the formula CnH2n–2. • Alkenes and alkynes are unsaturated; they contain fewer hydrogen atoms than alkanes.

6. Ethene (the Simplest Alkene) • Ethene has two sp2-hybridized C atoms • Each C has one unhybridized p orbital as well • The sp2 orbitals are in a trigonal plane, 120°apart.

7. 2pz Orbitals and the Pi (p) Bond • One p orbital on each C of ethene form a bond known as a pi bond. • Overlapping sp2 orbitals form a sigma (s) bond. • The p bond is formed at a 90°angle from the s bond.

8. 2pz Orbitals and the Pi (p) Bond • The figure shows p orbitals (blue) overlapping to form a pi bond and the sp2 orbitals (green) overlapping to form a sigma bond. A line bond structure of ethene is shown on the right. 8

9. Derivatives and Isomers of Alkenes • Derivatives of ethylene yield new compounds. • Replacing one H atom with some “X” group gives the vinyl compound.

10. Derivatives and Isomers of Alkenes (cont.) • Consider a simple alkene in which a methyl group and an X substituent are at either side of the double bond. There are two possible isomers of such a compound:

11. Derivatives and Isomers of Alkenes (cont.) • A cis- or Z- label is used for disubstituted alkenes like these in which the non-H substituents are pointing towards the same side of the C=C • A trans- or E- label is used for disubstituted alkenes like these in which the non-H substituents are pointing towards opposite sides of the C=C

12. Derivatives and Isomers of Alkenes (cont.) • A label is added prior to the name of the alkene to tell which isomer it is: • Demonstrating when and how these labels are used is a key purpose in this chapter.

13. Cis-Trans and E-Z Isomerism Similar groups on same side of double bond, alkene is cis Similar groups on opposite sides of double bond, alkene is trans E-Z nomenclature used when cis-trans no longer applicable. If high priority groups are on the same side, the name is Z (for zusammen). If high priority groups are on opposite sides, the name is E (for entgegen).

14. IUPAC Nomenclature • Parent is longest chain containing the double bond -ane changes to -ene (or -diene, -triene) • Chains are numbered to give the double bond the smallest possible number. • In a ring, the double bond is assumed to be between carbon 1 and carbon 2.

15. Nomenclature Examples 15

16. E-Z Nomenclature • Uses the Cahn-Ingold-Prelog rules to assign priorities to groups attached to each carbon in the double bond • Step 1-Use the priority system to distinguish atoms on the basis of atomic number. The atom of higher atomic number has the higher priority. • Step 2-For isotopes, atomic mass is used to break the tie in atomic number. • Step 3-Nonisotopic ties are broken by looking at the groups attached to the tied atoms. • Step 4-Multiple bonds attached to alkenes are treated as multiplied bonds.

17. E-Z Nomenclature (cont.) • Rule 1:

18. E-Z Nomenclature (cont.) • Rule 2: • Rule 3: Section 3.5

19. E-Z Nomenclature (cont.) • Rule 4: A special treatment is needed for substituents that have multiple bonds. We treat them as multiplied single bonds: • When prioritizing:As a substituent, the unit on the left is treated as the unit on the right Section 3.5

20. Alkene Stability: Heats of Formation (DHof) • DHof of a compound is the enthalpy of formation from its constituent elements in their standard states • Observe the pair of disubstituted alkenes; note the cis- and trans-3 hexene.

21. Cis- versus Trans- and Degree of Substitution • Cis-isomerism forces groups to eclipse each other (less stable; more positive DHof value). • The trans-isomer is the more stable compound (more negative DHof value). • More substituted alkenes are more stable.

22. Cycloalkenes • Rings larger than cyclopropene can have more than one double bond. • Cis-cycloalkenes are more stable than trans-cycloalkenes due to ring strain. • Trans-cycloalkenes are severely twisted and highly destabilized.

23. Cis- and Trans- Example

24. Natural Cycloalkenes • Many known cyclic and polycyclic compounds are found in nature.

25. Bredt’s Rule • A bridged bicyclic compound cannot have a double bond at a bridgehead position unless one of the rings contains at least eight carbon atoms. • The bridgehead position is the point at which the bridges meet.

26. Bicyclic Systems 26

27. Physical Properties of Alkenes • Differ only slightly from alkanes • Odor is more pungent (trivial name is olefins, referring to their smell) • In general, alkane mp and bp increase as molecular weight increases.

28. Physical Properties of Alkenes (cont.)

29. Alkynes • C-C triple bond is shorter than a C-C double bond, which is shorter than a C-C single bond • Greater s character leads to shorter bond • Internal alkynes are more stable than terminal alkynes

30. Relative Heats of Formation for Alkynes • DHf for alkynes much less stable than their constituent elements • The more p bonds in a molecule, the more positive the DHf.

31. Triple Bonds in Cyclics • These can occur in rings. • Undergo severe ring strain due to 180°linear geometry • Overlap between 2p orbitals of a p bond is reduced, forming the ring.

32. Strain in Cyclic Alkynes 32

33. Alkyne Physical Properties • Similar to alkanes and alkene

34. Alkyne Acidity • Alkanes/alkenes have high pKa (40–44) values. • Terminal alkynes differ; they are relatively strong proton donors. • An acetylideanion is produced by alkyne deprotonation • Alkynes are more acidic due to higher s character (50% s) • Greater s character = lower energy of electron = more stable anion

35. Degree of Unsaturation • DoU for a hydrocarbon = total number of p bonds and rings in the molecule

36. Alkene and Alkyne Addition Rxns • Pi bond acts as a proton acceptor and HX is the proton donor • H-X s bond is broken and new bonds are formed to the carbons of the old p system

37. HX Addition Mechanism • Alkene is protonated by acid first. • Halide addition occurs in the second step as soon as the carbocation is generated.

38. Regiochemistry of the Addition Rxn • Addition to either end of a symmetrical alkene generates no preference for the emerging carbocation made after alkene protonation. • Regiochemistry (a chemical distinction) is important when you have an unsymmetrical alkene.

39. Regiochemistry of Addition 39

40. Order of Carbocation Stability • More substituted carbocation is preferred upon alkene protonation.

41. Alkene Hydration • Alkene + acid catalyst  alcohol. • Alkene is protonated by H+ first, then water molecule attacks newly formed carbocation.

42. Hydration Mechanism 42

43. Study Guide for Chapter 3 SlidesSection(s)Topic 3-5 3.1 Preview 6-9 3.2 Alkenes: Structure and Bonding 103.3 Derivatives and Isomers of Alkenes 11-143.4 Nomenclature 15-163.5 The Cahn–Ingold–Prelog Priority System 17-183.6 Relative Stability of Alkenes: Heats of Formation 19-233.7 Double Bonds in Rings 243.8 Physical Properties of Alkenes 253.9 Alkynes: Structure and Bonding 263.10 Relative Stability of Alkynes: Heats of Formation 27-283.12 Triple Bonds in Rings 293.13 Physical Properties of Alkynes 303.14 Acidity of Alkynes 313.15 Molecular Formulas and Degrees of Unsaturation 323.16 An Introduction to Addition Reactions 333.17 Mechanism of Addition of HX 34-363.18 Regiochemistry of the Addition Reaction 37-383.19 A Catalyzed Addition to Alkenes: Hydration

44. Chapter 3 Checklist • Be Able To: • Describe the bonding model for alkenes and alkynes and sketch the orbitals involved • Name alkenes, including proper use of cis-/trans- and E-/Z- notation for geometric isomers • Use the Cahn-Ingold-Prelog Rules to assign priorities to substituents • Correlate stability of alkenes with structure in terms of electronic and steric arguments • Discuss how ring size and cis-/trans- isomerism dictate placement of multiple bonds in cyclic molecules • Use Bredt’s Rule to predict possible placement of multiple bonds in a polycyclic • Compare/contrast physical properties of alkanes, alkene, and alkynes • Discuss relative stabilities of alkenes on the basis of cis/trans isomerism and degree of substitution • Rationalize the acidity of alkynes compared to that of alkanes or alkenes • Predict products of addition reactions to alkenes (Addit. Of HX and Hydration) • Provide and discuss the mechanism of addition reactions and give rationale for observed regiochemistry • Do all problems at the end of Chapter 3 in your text; pay particular focus to any assigned homework: • By , you should be a master of Chapter 3 fundamentals.